Implantable cardiac stimulus devices, as well as many other implantable medical devices, typically include control circuitry that is adapted to perform various functions such as sensing, communication and/or stimulus delivery. Such devices operate within a patient's body, and are subject to various sources of electromagnetic interference (EMI) including, for example, noise from other electrical devices inside or outside of the patient's body, power line noise, noise generated by the patient's body itself, and, for some devices, noise that the device itself generates. For example, implantable cardiac stimulus devices typically deliver electric pulses to regulate or correct cardiac activity, and their sensing algorithms are often configured to avoid capturing self-generated signals. Some such devices, known as implantable cardioverter defibrillators (ICDs), deliver very large stimuli to shock a patient's heart out of an arrhythmic state such as ventricular tachycardia or ventricular fibrillation. When large pulses are delivered, it is desirable to limit the effects of the large pulse on operation of internal circuitry.
Certain devices must also be configured to dump charge. For example, during implant testing, an ICD may be required to deliver an induction signal to place a patient's heart in a desired arrhythmia (usually ventricular fibrillation), and then to detect and treat the induced arrhythmia. Once the induced arrhythmia is detected, the ICD will prepare for therapy by charging high power capacitors to a desired energy level. If the induced arrhythmia spontaneously terminates, however, for example by reverting to a normal sinus rhythm, the ICD will not deliver the therapy it has prepared itself for, and the induction test will be re-started. Prior to restarting the induction, the ICD must dump the high power capacitors safely. One approach is to use a dump resistor inside the device. The dumping operation must be performed relatively quickly, since the induction test generally takes place in the surgical suite and the patient and physician are both waiting. However, dumping cannot be done in a manner that causes excessive heating of the device, as heating beyond certain limits may be harmful to patients and is not allowed under internationally accepted device safety standards.
New and alternative designs for implantable medical devices are desired, and preferably such devices will address the above design needs.